Effects of alitude - power ?

[Ian_Vowles]

Does anyone have a suggestion for locating details for re-proping my vessel this winter. She is a 27SC with SP90 engine present prop produced 13-14 knots at sea level, at 6000 feet she runs 9.5 -10 knots at 2900 rpm - 3000 rpm ( WOT )

For interest in neutral she revs to 3600 rpm very nicely

Thoughts ?

Thanks

Ian

Miss Emily
1985 27SC
Navajo Lake
Colorado

[DougSea]

Welcome to AOG - I'm guessing you're one of very few "Mountain" members.

As to your question, due to your altitude you're no longer producing the HP that your engine is rated for at sea level. It seems to me your choices are; 1. Have the engine re-tuned to produce the proper HP or 2. Calculate your current HP and get a propeller shop to adjust/replace your prop.

For option 2 you can go to BoatDiesel.com and use their calculators to figure out your current effective HP. With that in hand contact one of the "Prop Scan" shops to get a propeller that allows your engine to turn it's full RPM's. (You could also re-post this question there - those guys know WAY more than I do about diesels and may have better suggestions / information) I don't think you'll pick up all of your speed, you are under-powered now, but by turning the proper RPM's you'll extend the life of your engine.

Hope that helps.

[Ian_Vowles]

Thanks Doug,

I'll investigate further - pitch on exisiting prop is 14 so I am considering that as well.

Again, thanks the response !

Snow on the hills....

Ian

[Mariner]

You've got several things working against you. First off, as Doug suggested, you're going to lose power from the engine at high altitude. There are things that can be done to combat this, but ultimately, the bottom line is that less oxygen=less power. Changing props will likely be necessary.

Also, you're switching from salt to fresh water. Propellers and boats behave quite differently in fresh water. The boat floats lower in fresh water, increasing your wetted surface. This effect is exaggerated at planing speeds, due to the shape of a planing hull. The largest factor you have to over come once you achieve a plane, is the drag from the wetted surface. Propellers have more "slip" in fresh water, and generally, a lower pitch is required to improve efficiency.

Lastly, transporting a boat over hundreds or thousands of miles of roadway provides all sorts of opportunities for something to be disturbed, dislodged, or damaged along the way. Most notable is the possibility for sediment to be stirred up in the fuel tanks and find it's way to the fuel filter, restricting flow, and reducing the engine's ability to make power.

I personally experienced something similar to this last summer. After a day on the water in Puget Sound, I pulled my Boston Whaler out of the water and immediately proceeded to drive it 700 miles across Washington State to Priest Lake, ID. Along the way, we experienced a massive rainstorm while crossing the mountains. Upon arriving in Idaho, I immediately launched the boat. It ran for about 30 seconds and then died. I spent hours trying to diagnose the problem, but couldn't, ultimately putting a significant damper on my weekend there. What I finally discovered was that at freeway speeds, heavy rain had been forced in large amounts into the fuel tank vents, contaminating the fuel with large amounts of water. Too much for the small fuel-water seperator to overcome. What's more, the bumpy gravel roads of rural Idaho had dislodged all sorts of sediment in the tank, as well as caused the carburetor float to stick and not perform right.

Road trips are tough on boats. A thorough going-over is often required before relaunching.

[Ian_Vowles]

Thanks Mariner,

Yes we used lost of filters in first 100 hours cleaning up distrurbed sediment.

I am inclined toward a two part solution a) tune engine to best we can b) change prop to a lower pitch.

I'll let you know how it works out in due course !

Ian